1. Field of the Invention
The present invention relates to the assembly of sheets to be fabricated into bent laminated windshields. Conventionally, bent laminated windshields for automobiles are fabricated using several separate steps. The first step involves bending a doublet, or a pair of glass sheets, into shapes that conform to the shape desired for the bent laminated windshield. The two sheets are bent as a pair so that their shape conforms to one another throughout the entire extent of the glass sheets. A tunnel-like lehr is used to heat the pair of bent glass sheets in unison to conform to the upwardly facing shaping surface of a bending mold.
Usually a parting material is applied to facilitate separating the sheets after the bending operation is completed. A particularly good technique for supplying the parting material involves spraying an aqueous suspension of fine diatomaceous earth in the manner disclosed in U.S. Pat. No. 2,725,320 to Atkeson and Golightly. The benefit of this patented process is that it is unnecessary to wash the parting material from between the interfacial surfaces of the matching bent glass sheets when the bent glass sheets are separated prior to assembling the sheets with a flexible sheet of interlayer material to form a sandwich.
A second step, which conventionally has taken place in an area remote from the glass bending lehr, involves the assembly of the pair of matched bent glass sheets with a flexible sheet of plasticized interlayer material to form the sandwich to be laminated. An interlayer for laminated safety glass windshields for automobiles presently used commercially is composed of a plasticized polyvinyl acetal resin, preferably polyvinyl butyral prepared in the manner recited in U.S. Pat. No. 2,400,957 to Stamatoff.
The polyvinyl butyrals contain a plasticizer. Generally, the plasticizer used is a water-insoluble ester of a polybasic acid and a polyhydric alcohol. Particularly desirable plasticizers for use with the interlayers assembled between pairs of bent glass sheets of matching curvature are triethylene glycol di(2-ethyl butyrate), dibutyl sebacate, di(beta-butoxyethyl) sebacate, dioctyl phthalate, and di-n-hexyl adipate.
Other interlayer materials include certain polyurethanes, preferably those derived by reacting an organic polyisocyanate with a material having a multiplicity of active hydrogen sites, such as a polyether polyol and/or a polyester polyol or a polyamine. U.S. Pat. No. 3,808,077 to Rieser and Chabal discloses polyurethanes suitable for use as the inner layer of bilayer windshields, which comprise an outer glass sheet and an inner layer of polyurethane. These polyurethane compositions are also useful as interlayers in conventional windshields of two glass sheets and an interlayer.
After the sandwich is assembled, it is laminated. In the manufacture of laminated windshields comprising a pair of matched sheets of bent glass and a thermoplastic interlayer, it is desirable to obtain a complete adhesive bond between the glass sheets and the thermoplastic interlayer. This complete adhesive bond is obtained commercially by subjecting the assembled glass and plastic sheets to heat and pressure while immersed in a hot oil bath while within an autoclave. Unless the marginal edges of the laminated assemblies are sealed by bonding the glass to the thermoplastic interlayer before the assembly is exposed to the oil bath, the oil in the autoclave may penetrate between the two sheets of glass and the interlayer. The assembly being laminated is discolored in the portion where oil penetration takes place.
In preparing assemblies for the autoclave, two bent glass sheets and a plastic interlayer are assembled in the form of a sandwich in an area remote from the bending lehr. The bent glass sheets are usually cooled to a temperature approaching room temperature during their transfer to an assembly area. Therefore, they form a cool sandwich of sheets that are able to slide relative to one another. To avoid sliding, the sandwich must be heated to a temperature at which the interlayer becomes tacky and resists relative sliding.
The heated sandwich is subjected to a preliminary pressing by passing the sandwich between one or more pairs of nipper rolls formed of resilient, yet frictional, contacting material that engage the outer surfaces of the assembly to be laminated. As the sandwich passes through the nipper rolls, air entrapped between the layers of the assembly is forced out. This preliminary pressing, sometimes followed by edge rolling of the margin of the plastic interlayer before the latter cools to room temperature, provides a sufficient seal extending over the entire marginal area of the assembly to prevent oil penetration and the resulting discoloration when the assembly is subjected to its final pressing operation in the oil autoclave. A typical roll pressing apparatus for prepressing glass-plastic sandwiches is disclosed in U.S. Pat. No. 3,351,001 to Achkio.
At times, some portion of the plastic sheet adjacent the edge of the laminate is not sealed to the glass after the prepressing operation, so that in the final pressing operation oil can enter the laminate. To alleviate this condition, any portions of the edge that are not sealed after prepressing are manually edge sealed to prevent the oil from entering the laminate during the final pressing operation. This edge sealing is accomplished immediately after prepressing by rolling and compressing the hot interlayer edge, so as to force it between the glass sheets into sealing contact therewith, with a thin metal disc known in the trade as an edge roller. A preferred edge roller is described in U.S. Pat. No. 2,999,779 to Morris.
The usual autoclave treatment utilizes temperatures ranging from about 190.degree. to 325.degree. F. (88.degree. to 163.degree. C.) preferably about 225.degree. to 300.degree. F. (107.degree. to 149.degree. C.) and, simultaneously, elevated pressure in the range of 100 to 250 pounds per square inch (0.7 to 1.7 megapascals) and preferably 175 to 225 pounds per square inch (1.2 to 1.5 megapascals) for a period of ten minutes to more than one hour, preferably 20 to 45 minutes. While oil autoclaves have been used more frequently in the past, final lamination has been accomplished by immersing prepressed assemblies in air autoclaves at approximately the same temperature and pressure conditions as those that prevail in oil autoclaves.
In the past, glass sheets were bent in pairs at one part of a windshield fabrication plant and assembled, prepressed, edge rolled and subjected to autoclave conditions in other parts of the plant. When sandwiches are assembled manually, it is convenient to allow the bent glass sheets to cool to a temperature suitable for handling before performing the sandwich assembly step. Such cooling wastes the residual heat in the bent glass sheets. It would be desirable for the assembly of the sandwich to take place at a time and place such that the energy presently lost through cooling the glass sheets between the bending operation and the assembly of the sandwich is not wasted so that the sandwich can be assembled, prepressed and edge-rolled using as much of the residual heat from the bending operation as possible. If the sandwich assembly is performed automatically, there is no need to be concerned with operator discomfort that relates to handling hot glass sheets.
It would be desirable to perform the sandwhich assembly step automatically rather than manually to provide a more consistent alignment of the sheets comprising the sandwich prior to their assembly while hot than is possible from assembling the sandwich by hand. When a flexible sheet of interlayer material is assembled between a pair of bent glass sheets at an elevated temperature at which the interlayer surfaces become tacky on contact with hot glass and is unable to slide relative to the hot glass it contacts, the sandwich remains in its originally assembled configuration during subsequent treatment steps including the prepressing operations using pressing rolls. Since automatic assembly provides more repetitive alignment than manual assembly, the chance of breakage due to misalignment of the bent glass sheets of the sandwich during roll pressing required for prepressing is minimized provided automatic assembly apparatus is provided with means to align the assembly position of the flexible sheet of interlayer material with those of the two bent glass sheets.
For the sake of this description, the term "bent glass sheet pair" refers to a pair of bent glass sheets without an interlayer sheet, the term "subassembly" is used to identify a partial sandwich of a single glass sheet and an interlayer sheet, the term "sandwich" comprises two matched bent glass sheets and a flexible sheet of interlayer material, the term "assembly" is applied to a sandwich that has been prepressed and is ready for final lamination, and the terms "laminate" and "laminated windshield" are applied to the finally laminated assembly after the latter is subjected to an autoclave operation.